We are studying the effect of endogenous beta cell secretory reserve on the metabolic control of diabetic patients. Beta cell function is assessed by measuring serum C-peptide levels in the basal state and following the administration of various secretagogues. Diabetic control is quantitated by hemoglobin A1 concentrations. If a positive relationship is found it will indicate the need to devise improved therapeutic approaches for preserving beta cell function in insulin dependent diabetics. The role of islet cell surface antibodies in insulin dependent diabetics is also being investigated. Their relationship to beta cell function at different stages of the diabetic syndrome is being monitored.